Study on Low-Frequency Abnormal Signal and Structural Characteristics of 2015 Azuoqi Ms5.8 Earthquake

In this article, the phenomenon of low-frequency abnormal signals before earthquakes, which reflects the three elements of earthquakes and the beneath structure change information, is discussed. Based on the data recorded at the Shizuishan (SZS), Wuhai (WUH) and Dongshenmiao seismic stations around the epicenter of the Ms5.8 earthquake in Azuoqi, Inner Mongolia, in 2015, the low-frequency abnormal signal from the seismic waves before this earthquake is extracted. At the same time, the autocorrelation method is used to extract the reflected waves of the main interface from teleseismic events recorded by the seismic array in the epicenter area, and then the change information from the beneath structure is obtained. It is explained in time and space that the low-frequency abnormal signal before the main earthquake, extracted from the continuous waveform, is directly related to the change in the underground structure near the epicenter, and it can be determined that the wave propagation direction f the crustal stress before the earthquake is from south to north, and it continues to accumulate near the epicenter until the main earthquake occurs.

Simulation of karst floods with a hydrological model improved by meteorological model coupling

Karst basins are prone to rapid flooding because of their geomorphic complexity and exposed karst landforms with low infiltration rates. Accordingly, simulating and forecasting floods in karst regions can provide important technical support for local flood control. The study area, the Liujiang karst river basin, is the most well-developed karst area in South China, and its many mountainous areas lack rainfall gauges, limiting the availability of precipitation information. Quantitative precipitation forecast (QPF) from the Weather Research and Forecasting model (WRF) and quantitative precipitation estimation (QPE) from remote sensing information by an artificial neural network cloud classification system (PERSIANN-CCS) can offer reliable precipitation estimates. Here, the distributed Karst-Liuxihe (KL) model was successfully developed from the terrestrial Liuxihe model, as reflected in improvements to its underground structure and confluence algorithm. Compared with other karst distributed models, the KL model has a relatively simple structure and small modeling data requirements, which are advantageous for flood prediction in karst areas lacking hydrogeological data. Our flood process simulation results suggested that the KL model agrees well with observations and outperforms the Liuxihe model. The average Nash coefficient, correlation coefficient, and water balance coefficient increased by 0.24, 0.19, and 0.20, respectively, and the average flood process error, flood peak error, and peak time error decreased by 13%, 11%, and 2 hours, respectively. Coupling the WRF model and PERSIANN-CCS with the KL model yielded a good performance in karst flood simulation and prediction. Notably, coupling the WRF and KL models effectively predicted the karst flood processes and provided flood prediction results with a lead time of 96 hours, which is important for flood warning and control.

The impact of heavy object on an underground structure when falling onto the ground surface

At the objects of space infrastructure and at nuclear power facilities there are industrial structures, the main task of which is to protect a person, equipment or machinery from emergencies such as, for example, explosions, falling of various objects, fragments. In accordance with the requirements of the Federal Law On the Protection of the Population and Territories from Natural and Technogenic Emergencies, when calculating such structures, all types of loads corresponding to their functional purpose must be taken into account. So, for structures located in the area of a possible accident and the fall of space rockets, it is necessary to calculate for the fall of the destroyed parts of the rocket engine. For nuclear power plant facilities, such accidents occur when containers and other heavy objects fall on the ground, affecting underground structures located in the ground, and for civil defense protective structures built into the basement floors of buildings, it is necessary to consider situations in which the overlying floors of a building collapse when exposed to there is an air shock wave on them. Therefore, this problem is relevant, and in this study, a finite-element method for calculating an underground structure in a non-linear dynamic setting has been developed when a large overall object collides with the ground.

MODELLING AND MONITORING OF STRUCTURAL SAFETY OF UNIQUE UNDERGROUND STRUCTURES OF THE SEWAGE SYSTEM OF LARGE CITIES IN DIFFICULT GROUND CONDITIONS

Anthropogenic and dynamic impacts on facilities of underground urban infrastructure increase at intensive development of megacities. The unique long-operating underground structures of sewage system require special protection against anthropogenic influence as their wear degree in difficult soil conditions reaches 70 % and more. Therefore, providing structural (mechanical) safety of underground structures of excessive level of danger and responsibility defines sustainable operation and future development of geotechnical infrastructure of the megacity in general. Long-term studying dynamics of changes of technical state of underground sewage structures of the megacity, long operating (for more than 70 years) in soft soils, allowed establishing regularities of influence of intensive anthropogenic and dynamic impacts on this process. For the first time, based on developed continuous models of defective structures potentially dangerous sections have been identified, they are subjected to manifestation of critical failures; ways of their correction are presented. Numerical simulation has defined borders of defectless joint operation of the system “target area - geomassif - underground structure". Scientific substantiation of boundaries of areas with potentially dangerous sections of underground sewage facilities with account of external anthropogenic and dynamic impacts constitutes the basis for elaborating regulations on safe development of geotechnical infrastructure of the historical area of St. Petersburg. The proposed methods of monitoring and protection of geotechnical infrastructure have been successfully used for many years by St. Petersburg Vodokanal in areas of influence of anthropogenic factors and objects under construction on underground structures, they ensure an optimal combination of sustainable operation and development of geotechnical infrastructures of megacities.

Sand–Tire Shred Mixture Performance in Controlling Surface Explosion Hazards That Affect Underground Structures

Blasting is an unavoidable activity in geotechnical engineering, road and tunnel construction, and mining and quarrying. However, this activity can expose the environment to various hazards that are challenging to control and, at the same time, critical for the safety of site workers, equipment, and surrounding structures. This research aims to evaluate the ability of sand–tire shred mixtures to reduce peak blast pressure, which is the leading cause of damage to underground structures under surface explosion. ABAQUS software is used to model the material behavior under explosion and is validated using the results of previous studies and an empirical equation. Different scenarios are created by using mixture layers with different thicknesses (2, 4, and 6 m) and tire shred contents (10%, 20%, and 30%) that are subjected to various surface explosion charges (100, 500, 1000, and 5000 kg). The thickness of the mixture layer is found to be directly related to the dissipation of explosion energy. However, the percentage of the rubber content in the mixture is only significant in reducing peak blast pressure when a thick enough mixture layer is used. The results confirm the adequate performance of the correctly chosen sand–tire shred mixtures in reducing peak blast pressure and protecting the underground structure from surface explosion hazards.

Application of an Automatic Noise or Signal Removal Algorithm Based on Synchrosqueezed Continuous Wavelet Transform of Passive Surface Wave Imaging: A Case Study in Sichuan, China

Passive surface wave imaging based on noise cross-correlation has been a research hotspot in recent years. However, because randomness of noise is difficult to achieve in reality, prominent noise sources will inevitably affect the dispersion measurement. Additionally, in order to recover high-fidelity surface waves, the time series input during cross-correlation calculation is usually very long, which greatly limits the efficiency of passive surface wave imaging. With an automatic noise or signal removal algorithm based on synchrosqueezed continuous wavelet transform (SS-CWT), these problems can be alleviated. We applied this method to 1-h passive datasets acquired in Sichuan province, China; separated the prominent noise events in the raw field data, and enhanced the cross-correlation reconstructed surface waves, effectively improving the accuracy of the dispersion measurement. Then, using the conventional surface wave inversion method, the shear wave velocity profile of the underground structure in this area was obtained.

Numerical analysis of shear interaction of an underground structure with soil

A soil layer behaviour under the shear interaction of an underground structure with soil is studied. Structural failure is considered under conditions of strained soil, and complete cohesion is assumed at the underground structure-soil contact boundary. The Finite Difference Method is used to numerically investigate the process of the structure-soil shear interaction under consideration. The main attention is paid to the adequacy of the conditions of soil-structure interaction, and to the strain state of the near-contact soil layer around the underground structure. The results are plotted and analysed. From the results obtained, the existence of a near-contact soil layer is shown; the use of the condition of complete cohesion is justified considering the structural failure of soil under conditions of complex interaction; the possibility to determine the thickness of the near-contact soil layer and of the layers with the respective degrees of structural failure is shown.

Seismic Analysis Method for Underground Structure in Loess Area Based on the Modified Displacement-Based Method

At present, the seismic design research of underground structures in loess areas is lagging behind compared with practical engineering requirements. The selection of seismic calculation methods and parameters does not consider the influences of the special geological conditions in various regions, so their usefulness is limited. Based on the above problems, a modified displacement-based method (DBM) was proposed and its application was compared with the most commonly used methods of analysis (force-based design method, displacement-based design method, detailed equivalent static analysis numerical method, and the full dynamic time-history method). The results were also validated by considering data from shaking table tests conducted on a case study involving the underground Feitian Road subway station in Xi’an. The results show that compared with DBM, the average accuracy of the modified DBM technique is improved by 41.65%. The modified DBM offers good accuracy, simplicity in its model, a rapid analysis time, and easy convergence.